Air born volatile organic and/or halogenic volatile organic compound contaminants (VOC's & HVOC's) and/or hydrocarbon compounds and/or fine suspended organic particulate particles (smoke), which may or may not be odorous—collectively referred to herein as airborne pollutants, are emitted into the environment air from a range of sources and processes and can fill the air in and about residential neighborhoods in still weather conditions. Many such airborne pollutants are considered to be pollutants and emission levels are regulated by the US EPA and even those that are not but are odorous can range in odorous delectability from mildly offensive to intolerable levels for the residents in the affected area. This is a common problem for residents that are in proximity to such sources. Examples of airborne pollutant emissions that are considered odorous sources include industries that process organic materials such as those that process and produce all types of food for human consumption which include bakeries, chocolate processing, frying and other related human food processing and industries that produce animal feed for the pet, fish, poultry and hog industry, and general agricultural applications. Other industries that process organic materials and release airborne pollutants that are odorous are those that process animal products including meat processing, fertilizer and rendering plants. Other airborne pollutant odor sources include composting facilities, sewage treatment centers, garbage transfer stations and other industrial organic processing facilities. Generally, these commercial and industrial operations exhaust gases from the preparation, handling, storage, cooking, grinding, drying, cooling, manufacturing, reduction and other related processes of organic materials. These exhausts contain medium to low-level mixed concentrations VOC's and HVOC's that include amines, aldehydes, and fatty acids that can be either fully evaporated and/or in aerosol form that are inherent in the materials processed, or resulting from the processing and are driven into the exhausted gas stream by the preparation activity. These industries typically have large gas flow volumes, ranging from 100 to 500,000 actual cubic feet of gas per minute (ACFM) and above.
Agricultural activities that raise animals for food production, such as hog, poultry and dairy farms also emit strong and offensive odors into the environment air from manure and barn ventilation odors and these can release offensive odors in sufficient quantity to fill many square kilometers under certain weather conditions.
Solvents, cleaning fluids, paint fumes, printed circuit board manufacturing and other VOC's and HVOC's are additional sources of environmental air emissions from other general industrial and commercial activities. Some VOCs may have little or no odor, but are considered atmospheric pollutants and/or carcinogens and need treatment to reduce them to harmless compounds prior to release into the environment. In the case where the airborne pollutants of concern are extremely odorous, even concentrations in the parts per billion ranges can be offensive or exceed environmental emission limits and these also need treatment.
There are various systems designed to oxidize and/or reduce VOC emissions and other airborne pollutants resulting from commercial and/or industrial processing activity that is to be emitted into the environment so that the emitted exhaust gas stream is within environmental regulatory limits. Some of these systems use non-thermal plasma (NTP) which is formed in dielectric barrier discharge (DBD) cells to create a wide range of activated species such as activated or reactive oxygen species (ROS), hydroxyl species and molecular and atomic species that are all at various levels of ionization and these are then mixed with the gas to be treated so that the VOC's and HVOC's undergo oxidation and/or reduction by the chemical reactions caused by the active species created by the NTP field. Many of the VOC's and HVOC's emitted from commercial and industrial processing, some of which are compounds that humans normally detect as odor, ultimately reduce to carbon dioxide and water vapor, though other products are possible depending on the chemical characteristics of the pollutants and by the energetic ions and species created in the non-thermal plasma field created by the DBD cells.
Activated species, as described herein, are chemical compounds and atomic species that are created in useful concentrations by the application of sufficient and appropriate electrical energy, such as through dielectric barrier discharge, to drive the molecules of interest from the ground state into the excited active states required, splitting some molecules into their atomic constituents, with the ground state being the normal state of these molecules typically at a nominal one atmosphere pressure and 20 degrees C. (or whatever atmospheric and temperature conditions occur at the place of the NTP field conditions). Activated species are typically designated in literature by “•” as in O• for active oxygen (atomic oxygen in this case). Activation occurs through a number of mechanisms including direct electron collisions or secondary collisions, light absorption, molecular processes involving ionization, or internal excitation.
Dielectric Barrier Discharge (DBD) technology is used to create the NTP that generates the activated species required for the purposes of this invention, and as such technology inherently limits the electron volt (eV) that can be applied to the gasses passing through the barrier, it is mainly the reactive oxygen species (ROS) which include a range of hydroxyl radicals and excited atomic oxygen, that are involved in this case, though other electron activity assists in the process. For the activated species generated in the NTP field, those ROS species that have the highest reduction potential (between about 2.4 and 6.5 eV) have the shortest availability with half-life concentrations of less than about 200 microseconds, such as in the case of oxygen singlet. These react with all compounds that pass through the NTP field, though in the case where the VOC's and/or HVOC's that need to be destroyed have a high ionic bonds, those need the high reduction potential active species that are also the shortest lived, found only in sufficient concentrations in the NTP field, to reduce and/or oxidize them. The highest reduction potential species, also called radicals, and the reactions between them and the VOC and/or HVOC compounds needing a high level of eV energy to destroy them, occur only in the NTP field, as the most active radicals quickly decay to less active species outside the NTP field. The radicals react with the VOC and/or HVOC compounds by oxidation and reduction transformations so that the VOC compounds are transformed to simpler molecular compounds that are no longer detectable as odor and are no longer classified as pollutants. Additional activity occurring within the NTP is that of electron collisions, bombardment and direct ionization, which acts on all molecules within the field, including the compounds of concern. This electron action, as well as creating the ROS of interest, also results in the disruption of the molecular bonds of the VOC and/or HVOC compounds, which also aids in the ROS activity of oxidation and/or reduction of the VOC and/or HVOC compounds reducing them to non odorous forms. The NTP field also creates, within the ROS, a range of lower reduction potential radicals (between about 1.4 and 2.4 eV), and these are longer lived with half-lives from about 100 milliseconds to several minutes at normal atmospheric temperatures and pressures. These radicals react with the VOC and HVOC compounds that respond to this level of reduction potential and oxidation for decomposition. These reactions occur both in the NTP field and in the air stream outside the NTP field, as those radicals are active longer, and are carried outside the NTP field by the airflow through the DBD. These longer-lived radicals also effect their changes on the VOC and HVOC compounds by oxidation and reduction transformations, so that the compounds of concern are transformed to simpler molecular forms that are no longer considered to be pollutants or detectable as odor. Such transformations also ultimately convert the complex organic molecules and hydrocarbon molecules into the most simplified oxides, such as carbon dioxide, hydrogen dioxide (water), nitrogen (N2) and other simplified molecular or oxide forms of the elements that were in the original complex compounds. In cases where large volume air streams need to be treated, usually a portion of the air to be treated passes through the NTP field for treatment and excessive ROS, hydroxyl, ionized molecular and atomic activation and these species created is then immediately mixed with the balance of the air to be treated so that the treatment of the entire air stream is accomplished. In cases where the pollutant of concern is relatively highly concentrated or simply needs more energy than what a single NTP field is able to deliver, then the air path can be through multiple series and/or series parallel NTP fields to effect treatment.
Four oxidation states of molecular dioxygen are known: [O2]n, where n=0, +1, −1, and −2, respectively, for dioxygen, dioxygen cation, superoxide anion, and peroxide dianion (symbolically expressed as 3O2, 3O2.+, 3O2.−, and 3O2−2). In addition, “common”oxygen in air, 3O2, is in a “ground” (not energetically excited) state. It is a free “diradical”having two unpaired electrons. The two outermost pair of electrons in oxygen have parallel spins indicating the “triplet” state (the preceding superscript “3”, is usually omitted for simplicity). Oxygen itself is a common terminal electron acceptor in biochemical processes. It is not particularly reactive, and by itself does not cause much oxidative damage to organic compounds. It is a precursor, however, to other oxygen species that can be toxic, including: superoxide anion radical, hydroxyl radical, peroxy radical, alkoxy radical, and hydrogen peroxide. Other highly reactive molecules include singlet oxygen, 1O, and ozone, O3.
Ordinary oxygen does not react well with most molecules, but it can be “activated” by the addition of energy (naturally or artificially derived; electrical, thermal, photochemical or nuclear), and transformed into reactive oxygen species (ROS). Transformation of oxygen into a reactive state from the addition of a single electron is called reduction (Eqn. 1). The donor molecule that gave up the electron is oxidized. The result of this monovalent reduction of triplet oxygen is superoxide, O2•−. It is both a radical (•, dot sign) and an anion (charge of −1). Some reactive oxygen species known to be created with NTP, but by no means all, are noted below:O2+e→O2•−  (Eqn 1)2O2.−+2H+→H2O2+O2•  (Eqn 2)O2.−+H2O2→O2+OH.+OH−  (Eqn 3)O2.−+2H2O→O2+HO2.−+OH•−  (Eqn 4)2O2.−+O2+H2O→2O2+OH−+OH•  (Eqn 5)
For any given reactive oxygen species (ROS), there exists some confirmed or postulated reaction scheme for inter conversion to any of the other species. In any event, several of the above reactive oxygen species may be generated in the NTP and react with the VOC and/or HVOC compounds to transform them into simpler compounds that are no longer considered to be pollutants or detected as odorous.
Commercial and industrial volumes of air and/or gases to be treated normally have contaminants such as condensing water or other vapors and liquids, particles of some kind, or mixtures of both condensing fluids and particles. A problem arising from the use of dielectric barrier discharge (DBD) cells, generating the NTP for treating industrial scale flows of contaminated gases, is that after a period of use, the contaminants inherent in these gases build up in the cells and cause electrical short circuits in the cells from hot electrodes, across the insulation and support frames, to the ground frame or ground electrodes. This interferes with the designed electrical properties of the DBD cell and immediately destroys any ability for the DBD cell to generate the NTP should a continuous arc occur or the integrity of the DBD insulation becomes compromised. In some cases, a quick flashover arc will occur and clear. An industrial example of this is in utility electrical power distribution, where the porcelain or more modern type, polymer concrete insulators on the high tension lines can withstand a short duration electrical flash. In the case where a sustained arc occurs it is very likely DBD cell component damage has occurred as electrical arcs have very high temperatures and parts are usually damaged that have been in contact with the arc, and at the very least, cleaning of the DBD cell is necessary to restore the electrical dielectric integrity of the DBD cell, and damaged parts must be replaced. It is an important, practical consideration to pay due attention to the DBD cell design so that the effects of normal operation in an industrial environment have minimal effect on the operation of the DBD cells and no combustible material be used in any area of the cell where a flash over might occur.